The present invention relates to an optical recording medium, method for optical recording by using the same, method for reproducing optical signals recorded thereon as well as an apparatus for optical recording by using the same and apparatus for reproducing optical signals recorded thereon.
In recent years, several types of optical recording media suitable for recording and reproduction of information signals in a high density at a high speed have been developed and are now widely employed in the information processing technology. The optical recording media now under practical applications as a rewritable optical recording medium include those of the magnetooptical (MO) recording type by utilizing the interaction of light with the magnetic behavior of the recording medium called the Kerr effect or Faraday effect, those of the phase-change (PC) type which utilizes the difference in a certain optical property such as transmissivity and reflectivity between an amorphous phase and a crystalline phase in an alloy composed of a chalcogen element. Besides, optical recording media of the writing-only type (WO) have already been commercialized. In this type of recording, the recording film contains an organic dye which is thermally decomposed by the heat of light to utilize the difference of the optical properties caused thereby for recording.
In these three types of optical recording media, the recording density is increasing year by year in recent years to comply with the requirements for accomplishing a more and more information-prevailing society. Very active investigations are now under way for the development of rewritable DVD-RAMs and non-rewritable DVD-Rs.
It is taken as prospective that the optical recording media of the phase-change type are the most suitable, among the three types of the recording media mentioned above, for high-density recording of information signals by virtue of the properties of the alloy used therein and various methods have been developed in this category. For example, a very high recording density of 15 gigabytes is already accomplished for a single side of a 12 cm-size disk of this type by combining a blue laser beam with a specific alloy as the recording medium as reported by Kitaoka, et al. in 1997 (Ninth Symposium on Phase-Change Recording, page 94). Further, a proposal is made by Hosaka, et al. in Japanese Journal of Applied Physics, volume 35 (1996), page 443 for a recording technology to accomplish a still higher recording density by utilizing the difference in the optical properties between two states of the recording layer induced by the phase change when conversion is effected from the as-deposited amorphous state into a crystalline state. By virtue of utilization of the near-field optical recording, a success has already been attained in this recording technology to accomplish a recording dot mark having a radius of as small as 60 nm to 200 nm. According to this report, however, a grain radius smaller than 60 nm could not be observed in the phase-change films. This is presumably because the activation energy accompanying the crystal growth was so large that the recording power was not large enough for the conversion in this method from the as-deposited state with randomness to a crystalline form of GeSbTe. Further, Sumi, et al. attempted recording on a phase-change recording film by utilizing an atomic force microscope (Japanese Journal of Applied Physics, volume 36, 1997, page 523). As a result, a charge distribution could be obtained by the Schottky contact between the recording film and the chromium-coated head of the atomic force microscope leading to the possibility of recording. Reportedly, a dot mark having a diameter of about 10 nm could be successfully recorded. Since the head of an atomic force microscope is used in the latter case, however, reproduction could not be performed for the recorded dot marks of 10 nm or smaller obtained in optical recording.
In connection with the high-density recording by the use of a near-field light or atomic force microscope, all of the reports available so far for reproduction of the recorded signals are directed to the experiments under a microscope and no reports are available for reproduction of recorded signals at a high transfer rate. This is presumably due to the distance between the head for recording or reproduction and the recording medium. In the case of the near-field recording, on the other hand, the propagating distance of the near-field light is so short as to be about 50 nm so that crashing may eventually take place between the recording medium and the head moving at a high speed over the recording medium when reading-out of the recorded data is performed at a high transfer rate to destroy the recorded data. This situation is the same also in the use of an atomic force microscope and it is impossible to control the distance between the head and the recording medium with an accuracy of a nanometer order. Due to these technological difficulties, high-speed recording and high-speed reading-out have not been accomplished heretofore in the high-density recording utilizing a near-field light.